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The ratio of the line spacing along 100 to the line ... the first-order lines for the MgO substrate Fig. ... line crossing the step, one can see that the atomic rows on.
APPLIED PHYSICS LETTERS

VOLUME 77, NUMBER 16

16 OCTOBER 2000

Molecular beam epitaxial growth of atomically smooth scandium nitride films Hamad Al-Brithen and Arthur R. Smitha) Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701

共Received 31 May 2000; accepted for publication 24 August 2000兲 High quality scandium nitride films have been grown on magnesium oxide 共001兲 substrates by molecular beam epitaxy using a rf plasma source for nitrogen. Both reflection high energy electron diffraction and x-ray diffraction confirm that these films have 共001兲-orientation. Atomic force microscopy reveals a surface morphology consisting of large plateaus and pyramids. The plateaus are found to be atomically smooth and have a 1⫻1 surface structure, as revealed by in situ scanning tunneling microscopy. © 2000 American Institute of Physics. 关S0003-6951共00兲00142-X兴

The growth and surfaces of nitride semiconductors have recently been subjects of great interest.1–4 Most of the work has concentrated on group XIII nitrides, and GaN in particular. Yet there are other nitrides with interesting properties, an example of which is scandium nitride, a group III transition metal nitride. For this unusual material, there is evidence that it is a semiconductor having a direct band gap in the range 2.1–2.4 eV.5–10 Different from most conventional semiconductors, ScN is known to stabilize in the rock-salt crystal structure.5,7,9 Due to the strong bonding between Sc and N, ScN is also thought to have a very high melting point of over 1500 °C. 7,11 Finally, ScN has a very small lattice parameter mismatch with GaN (⬍0.3%), which might allow the growth of GaN/ScN heterostructures or ScGaN alloys.6 For a potentially useful electronic material, it is important to demonstrate that smooth epitaxial growth of singly oriented films can be achieved. Dismukes et al. grew ScN using chemical vapor deposition on sapphire, resulting in 共111兲-oriented, but rough, films.5 Moustakas grew ScN on sapphire共0001兲 using electron cyclotron resonance 共ECR兲 molecular beam epitaxy 共MBE兲, which also resulted in 共111兲-oriented, but rough, films.9 Gall et al. grew ScN on MgO共001兲 using reactive magnetron sputtering, resulting in films having both 共001兲 and 共111兲 orientation.7 A subsequent paper by Gall et al. reported that the use of a 20 V substrate bias during growth resulted in single 共001兲 orientation but that these films were also rough.8 In this letter, we report the smooth growth of ScN using radio frequency 共rf兲 MBE. We find that rf MBE growth results in well-oriented ScN films, having either 共001兲, 共110兲, or 共111兲 orientation, depending on the starting substrate orientation. We find that 共111兲 and 共110兲 oriented ScN films grow in a 3D growth mode with rough surfaces, but 共001兲oriented ScN films can be grown in a 2D growth mode, resulting in atomically smooth surfaces. The experiments are performed in a custom-designed vacuum system consisting of a MBE chamber coupled to a surface analysis chamber. The substrates are first cleaned with solvents, then loaded into the MBE chamber and heated up to ⬃1000 °C for 30 min 共for growth on sapphire, the a兲

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nitrogen plasma is also applied during this heating step兲. Then the sample temperature is lowered to ⬃800 °C prior to beginning the growth of ScN. The nitrogen flow rate is 1.1 sccm with the rf power set at 500 W. The effective Sc flux, estimated from the measured film thickness and the growth time, is in the range 4⫻1013 – 3⫻1014/cm2 s. The growth condition is monitored using reflection high energy electron diffraction 共RHEED兲. Following growth, the sample is analyzed by in situ scanning tunneling microscopy 共STM兲. After removal from the surface analysis chamber, the sample is analyzed using x-ray diffraction 共XRD兲 and atomic force microscopy 共AFM兲. Initially, we grew ScN on sapphire 共0001兲 substrates, resulting in 共111兲-oriented ScN. While the RHEED patterns during growth showed good ordering along the high symme¯ 0 典 and 具 11 ¯ 00典 , the patterns were very try directions, 具 112 spotty, indicating a rough growth surface. The roughness of these surfaces was later confirmed in AFM images, and XRD confirmed the 共111兲 orientation. We also tried growth on MgO共110兲, and although the RHEED patterns showed good 共110兲 orientation, they were also very spotty. Evidently, ScN takes on the crystalline orientation of its substrate, but smooth growth is difficult 共under our MBE conditions兲 for 共110兲 and 共111兲 orientations. This could be due to very small adatom diffusion lengths on surfaces with these orientations, as was suggested by Gall et al. in the case of 共111兲 orientation.7,8 If adatom diffusion lengths are larger for the 共001兲 surface, then smooth growth may be possible. To grow 共001兲oriented ScN, we use MgO共001兲 as a substrate. MgO also has rock-salt structure, and the lattice mismatch of ScN with MgO is 7.3%. The RHEED patterns shown in Fig. 1 illustrate the stages of the growth process. Prior to heating, the as-loaded substrate has good crystalline quality, as seen in Fig. 1共a兲, which shows the RHEED patterns along the 关100兴 and 关110兴 azimuths. Clear diffraction spots are seen along both azimuths, as well as Kikuchi lines. However, the patterns are spotty, indicating some roughness. After heating the substrate to 1000 °C for 30 min, the diffraction spots elongate and sharpen into the distinct streaks shown in Fig. 1共b兲. These streaky patterns suggest that the MgO surface may be smoothened by the heating.

0003-6951/2000/77(16)/2485/3/$17.00 2485 © 2000 American Institute of Physics Downloaded 23 Aug 2002 to 132.235.22.111. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

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Appl. Phys. Lett., Vol. 77, No. 16, 16 October 2000

H. Al-Brithen and A. R. Smith

FIG. 2. 2␪ x-ray diffraction spectra for two different ScN films grown on MgO共001兲. Neither film shows any 共111兲 peaks.

FIG. 1. Sequence of RHEED patterns for ScN growth process on MgO共001兲. 共a兲 As-loaded MgO surface prior to heating; 共b兲 MgO surface after 30 min of heating at 1000 °C; 共c兲 ScN共001兲 surface during growth.

Scandium nitride growth is initiated on this smoothened MgO共001兲 surface. The RHEED patterns show good epitaxy from the very beginning. Figure 1共c兲 shows the RHEED patterns during growth for a ScN film of thickness about 1200 Å. These patterns are fairly streaky, indicating smooth growth. The ratio of the line spacing along 关100兴 to the line spacing along 关110兴 is close to &, indicating four-fold surface symmetry. Detailed analysis of the RHEED patterns can be used to estimate the in-plane lattice constant of the film surface. This is done by dividing the spacing between the first-order diffraction lines for the ScN 关Fig. 1共c兲兴 by the spacing between the first-order lines for the MgO substrate 关Fig. 1共b兲兴. For the 1200 Å thick sample, we get a ratio of about 1.04, 2.5% less than the ratio of the expected bulk lattice constants for ScN and MgO at 800 °C, 1.065.5,8 For thicker films 共⭓2000 Å and grown with higher Sc flux兲, we have measured ratios which are very close to the expected bulk ratio. Figure 2 shows XRD spectra for two films—one about 1200 Å thick and the other about 2400 Å thick. The two spectra have been normalized to the height of the ScN 共002兲 peak which occurs near 40°. The peak in each spectrum near 43.1° is the MgO 共002兲 peak. No ScN 共111兲 peaks 共which would be at 34.5°兲 are observed for these or for any of our films grown on MgO共001兲. We conclude that rf-MBE growth on MgO共001兲 results in single oriented ScN共001兲. By measuring the positions of the ScN 共002兲 peaks in comparison to the MgO 共002兲 peaks, we calculate the perpendicular lattice constants for these two films. In the case of the thinner film, we get a value of 4.48 Å, slightly smaller than the expected value of 4.501 Å. For the thicker film, the ScN共002兲 peak gets closer to 40.0°, giving a perpendicular lattice constant of about 4.51 Å, slightly larger than the expected bulk value.

Atomic force microscopy images of the ScN共001兲 films clearly reveal a plateau-pyramid morphology, as shown in Fig. 3. For this 1 ␮ m⫻1 ␮ m image, one observes numerous plateaus with square shapes as well as many pyramids which are four-sided. The edges of these plateaus and pyramids are along 具 100典 directions of the film which also coincide with the 具 100典 directions of the substrate. The pyramids have the same 共001兲-orientation as the plateaus. If they were 共111兲-oriented, we would expect to see three 共001兲 facets and a 共111兲 peak in XRD—but we do not. In fact, the sides are gently sloping, with typical apex angles of about 165°. Moreover, STM images reveal closely spaced steps on the sides of the pyramids. We believe the pyramids are centered on dislocations in the film. Assuming this, and counting the number of pyramids within the 1 ␮ m2 image of Fig. 3, we estimate a dislocation density of ⬃109 /cm2. As seen in Fig. 1共c兲, the RHEED patterns for ScN共001兲 show only 1⫻1 symmetry. This is expected since ScN has the rock-salt structure, and therefore the ionic character of the bonding will tend to suppress charge transfer, thus pre-

FIG. 3. Atomic force microscopy image of ScN共001兲 grown on MgO, illustrating the plateau-pyramid morphology. The gray scale range is 89 Å. Downloaded 23 Aug 2002 to 132.235.22.111. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp

Appl. Phys. Lett., Vol. 77, No. 16, 16 October 2000

FIG. 4. Scanning tunneling microscopy image of plateau region on ScN共001兲. The image was acquired at a sample bias of ⫺0.5 V and a tunneling current of 0.08 nA. The enhancement at the step edge is due to a local background subtraction. The inset is an expanded view of the surface acquired with a sample bias of ⫺1.0 V and a tunneling current of 0.2 nA. A model of the rock-salt surface lattice is fit to the data.

venting reconstructions. Shown in Fig. 4 is a STM image of the ScN surface showing atomic resolution. The square 1⫻1 periodicity is clearly evident. The inset shows a magnification of the surface with a simple overlay for the rocksalt lattice. The larger square indicates the conventional surface unit cell, having an atom at the face center and sides along 具 100典 . The smaller square inside, rotated by 45°, is the primitive surface unit cell whose sides are aligned with the atomic rows observed in the image. Therefore, the atomic rows in the image are along 具 110典 . The two terraces in the image are separated by a single step. The measured height of this step is very close to 2.25 Å,12 or half the lattice constant. By sighting along the dashed line crossing the step, one can see that the atomic rows on the lower terrace are shifted by half the row spacing along the 关110兴 direction compared to the atomic rows on the upper terrace. This shift is due to the fact that adjacent 共001兲 planes of the rock-salt lattice are offset by a/2 along the 关100兴 direction. A single 共001兲 lattice plane of the rock-salt structure contains an equal number of N and Sc atoms. If we assume that the surface structure is similar to the bulk truncation, then our STM image should correspond to one of the two sublattices 共Sc or N兲. Since the STM image of Fig. 4 was acquired with a sample bias of ⫺0.5 V, one possibility is

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that we are imaging the N atoms 共filled states兲. However, dual bias images obtained at ⫹1 and ⫺1 V showed only a small relative shift, much less than the expected a/2. More work is needed to clearly distinguish the two expected sublattices. While this letter has not presented data on the optical properties of these MBE-grown ScN films, from visual inspection, the films have a reddish or rusty color when white light is shined through them, consistent with an absorption near 2.2 eV. Luminescence and optical transmission measurements are currently being performed to investigate in detail the optical properties. In conclusion, we have investigated the growth of ScN by rf MBE. We find that well-oriented ScN films can be grown which take on the orientation of their substrate. While 共110兲 and 共111兲 orientations have rough surfaces, 共001兲oriented ScN can be grown smooth, which has been shown by RHEED, AFM, and STM data. For growth on MgO共001兲, we find only 共001兲-oriented ScN. We have shown atomically smooth terraces separated by single steps of height 2.25 Å, half the lattice constant of ScN. Work is underway to clarify the detailed surface structure, to understand the dependence of the film properties on the Sc flux during growth, and to measure the optical properties of these films for different MBE growth conditions. The authors gratefully acknowledge the contributions of Megan Krejny and Marcus Legros for their construction of essential equipment used in this study, and Bethany Revill, who helped with AFM imaging and cataloguing of some of the data. The authors also thank the Office of Naval Research for supporting this work. 1

A. R. Smith, R. M. Feenstra, D. W. Greve, J. Neugebauer, and J. E. Northrup, Phys. Rev. Lett. 79, 3934 共1997兲. 2 A. R. Smith, R. M. Feenstra, D. W. Greve, M.-S. Shin, M. Skowronski, J. Neugebauer, and J. E. Northrup, Surf. Sci. 423, 70 共1999兲. 3 M. H. Xie, S. H. Cheung, L. X. Zheng, Y. F. Ng, Wu Huasheng, N. Ohtani, and S. Y. Tong, Phys. Rev. B 61, 9983 共2000兲. 4 S. Yu. Karpov, R. A. Talalaev, Yu. N. Makarov, N. Grandjean, J. Massies, and B. Damilano, Surf. Sci. 450, 191 共2000兲. 5 P. Dismukes, W. M. Yim, and V. S. Ban, J. Cryst. Growth 13Õ14, 365 共1972兲. 6 J. P. Dismukes and T. D. Moustakas, Proc.-Electrochem. Soc. 96-11, 111 共1996兲. 7 D. Gall, I. Petrov, L. D. Madsen, J.-E. Sundgren, and J. E. Greene, J. Vac. Sci. Technol. A 16, 2411 共1998兲. 8 D. Gall, I. Petrov, N. Hellgren, L. Hultman, J. E. Sundgren, and J. E. Greene, J. Appl. Phys. 84, 6034 共1998兲. 9 T. D. Moustakas, R. J. Molnar, and J. P. Dismukes, Proc.-Electrochem. Soc. 96-11, 197 共1996兲. 10 Whether or not ScN also has an indirect band gap at lower energy is currently an issue under investigation. 11 W. Lengauer and P. Ettmayer, J. Less-Common Met. 168, L7 共1991兲. 12 We calibrated the z scale of our STM by measuring single steps heights on ¯ ) grown on sapphire, which are known to have a height of GaN(0001 2.59 Å.

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